In the compound solar cells such as CIGS (Copper-Indium-Gallium-Selenium), CIS (Copper-Indium-Selenium), CdTe (Cadmium-Tellur), or the like, the thin film solar cells such as amorphous Si or the like, the hybrid solar cells in which a plurality of the solar cells are layered, and the organic EL (electroluminescence) illuminations, foundations called base materials are utilized in order to structurally support the CIGS layer, the CIS layer, the CdTe layer, the amorphous Si layer, the organic EL layer, or the like.
Conventionally, as described in Patent Document 1, glass base materials are frequently utilized as the base materials. However, since the glass is fragile, the glass base materials need to be thickened in order to ensure predetermined strength. Thickening the glass base materials results in an increase in weight of the solar cells and the organic EL illuminations in itself.
On the other hand, metal foils are recently tried to be utilized as the base materials instead of the glass base materials. The metal foil is not fragile, and can be suitably thinned. It is required for the metal foils utilized as the base materials that all of corrosion resistance, surface smoothness, and elastoplastic deformability are excellent.
The corrosion resistance is required in order to be capable of exposing the metal foils utilized as the base materials to an outdoor environment for prolonged periods which are considered to be 20 years.
The surface smoothness is required in order to prevent the solar cell layer or the organic EL layer which is layered on the base material from being physically damaged by protruding defects which exist on a surface of the base material. It is desirable that the surface of the base materials is a smooth surface which does not include the protruding defects.
The elastoplastic deformability is required in order to be capable of coiling the metal foil for the base material so as to be a roll shape, which is impossible by using the glass base materials which are hard. As a result, if production by a batch processing is changed into continuous production by a Roll-to-Roll processing, a production cost for the solar cells and the organic EL can be drastically reduced.
In general, as the metal foils for the base materials, stainless steel (SUS) foils which are excellent in the corrosion resistance are tried to be utilized. As described in Patent Document 2, the base materials in which an organic film is further formed on the SUS foils may be utilized.
Since the SUS foils are excellent in the corrosion resistance, the SUS foils are utilized as the metal foils for the base materials. However, there is a problem in that the SUS foils are expensive as materials. Moreover, since the SUS foils are hard and are not readily subjected to a rolling process, there is a problem in that the production cost is expensive. Thus, the utilization thereof is not pervasive in the present circumstances as compared with the glass base materials.
On the other hand, since plain steel (carbon steel) foils are inexpensive as the materials as compared with the SUS and have excellent deformability, the production cost can be drastically reduced. However, in case of the plain steel foils in itself, the corrosion resistance which is required as the metal foils for the base materials cannot be satisfied. If the plain steel foils which satisfy the above-mentioned properties required as the metal foils for the base materials are utilizable, the production cost for the solar cells and the organic EL can be drastically reduced. Therefore, development thereof is eagerly anticipated at present.